This invention relates to alkaline cells in general and more particularly to a flat alkaline cell construction wherein the active elements are enclosed within a sealed, liquid-impervious plastic film envelope and wherein the envelope is provided with both positive and negative terminal connections and a third terminal connection for an auxiliary electrode. In one aspect of the invention, a flat rechargeable cell enclosed within a sealed plastic film envelope and having positive and negative terminal connections is provided with a third terminal connection for an oxygen or hydrogen sensing electrode or a voltage reference electrode.
In our copending application Ser. No. 456,838 entitled "Flat Alkaline Cell Construction And Method For Assembling The Same", filed Apr. 1, 1974, of which this application is a Continuation-In-Part, there is disclosed and claimed a flat alkaline cell wherein at least a pair of flat electrode elements of opposite polarity having a porous separator containing the alkaline electrolyte interposed therebetween are arranged in the form of a conventional electrode stack assembly. This electrode assembly is enclosed within a sealed, liquid-impervious plastic film envelope having a terminal connection provided in one wall thereof adjacent to one end of the electrode assembly. The terminal connection is made using both internal and external current collectors which are disposed adjacent to the inner and outer sides respectively of the wall of the sealed envelope and which overlie an opening in the wall. The internal current collector is disposed adjacent to the end of the electrode assembly and is in electrical connection with one of the electrode elements. A thin layer of an adhesive sealant is interposed between the internal current collector and the inner surface of the wall of the sealed envelope and another thin layer of the adhesive sealant is interposed between the external current collector and the outer surface of the wall. Both layers of adhesive sealant cover substantially the entire face of both current collectors, except for a small area coinciding with the opening in the wall. Suitable means are provided for making electrical connection between the internal and external current collectors through the opening, such as a spot weld.
Alkaline electrolytes are notoriously known for their ability to readily wet most metal and plastic surfaces and to creep past seals conventionally used in current-producing electrochemical cells. Our flat cell construction as described above effectively solves this problem through the use of an adhesive sealant which is non-wettable by the alkaline electrolyte. The thin layers of adhesive sealant used to tightly bond both current collectors to the plastic film envelope actually resist creepage of the electrolyte past the sealing interfaces and out through the opening in the wall of the envelope. Preferably, the adhesive sealant is chosen from the class of compounds known as "fatty polyamides", although other adhesive sealants which are not readily wet by the alkaline electrolyte can also be used.
Still another advantage of our flat cell construction resides in the provision of an extended leakage path over which the electrolyte must travel in order to escape from the cell. This extended leakge path comprises both of the sealing interfaces between the internal and external current collectors and the inner and outer sides of the wall of the sealed envelope or in other words, the leakage path traverses approximately the total width or length of each collector.
In a typical flat cell construction made in accordance with our above referred to copending application, the sealed envelope is advantageously formed by a pair of plastic film wall members arranged such that each wall member overlies an end of the electrode assembly, both plastic film wall members being sealed together along their marginal borders such as by a heat seal. A positive terminal connection is provided in one of the plastic film wall members adjacent one end of the electrode assembly and a negative terminal connection is provided in the other plastic film wall member adjacent to the other end of the electrode assembly. Both positive and negative terminal connections are made in the same manner as described hereinabove using internal and external current collectors tightly adhered to the plastic film wall members by a thin layer of the adhesive sealant. The arrangement of the terminal connections is such that a plurality of the individual flat cells may be stacked together with the positive terminal connection of one cell being in electrical connection with the negative terminal connection of an adjacent cell in the stacked assembly to constitute a series-connected battery. Flat cells with this arrangement of the terminal connections can also be assembled into parallel and series-parallel connected batteries.
It has already been proposed in the prior art to employ so-called "auxiliary electrodes" in current-producing electrochemical cells. These auxiliary electrodes are used in conjunction with the working electrodes, i.e., positive and negative electrodes, to perform certain valuable functions in operation of the cells. Such auxiliary electrodes have been used, for instance, in rechargeable cells to detect or sense the presence of excessive amounts of either oxygen or hydrogen gas that may be generated under certain conditions such as when the cells are overcharged.
In U.S. Pat. No. 3,462,303 to H. Reber, there is disclosed a sealed rechargeable cell wherein an auxiliary electrode is maintained in contact with a gas space and the liquid electrolyte. The auxiliary electrode will form with the negative electrode of the cell a voltage differential the value of which will be dependent on the partial oxygen pressure in the gas space of the sealed cell. When the cell is subjected to overcharging, the partial oxygen pressure in the gas space will rise, a change in the voltage differential will occur and this change is utilized for actuating control devices for terminating the charging current and thereby prohibiting the build-up of an excessive gas pressure inside the cell.
Basically the same auxiliary electrode arrangement may be used in a rechargeable cell such as described above to sense the pressure of hydrogen gas pressure in the cell. In this instance, the auxiliary electrode which will form with the positive electrode of the cell a voltage differential whose value will be dependent on the partial hydrogen pressure in the gas space under conditions where hydrogen gas may be evolved during operation of the cell.
Auxiliary electrodes may also be incorporated in current-producing electrochemical cells as a voltage reference device. It is possible for instance to electrochemically couple the positive or negative electrode of a rechargeable nickel-cadmium cell to an auxiliary reference electrode and study the discharge behavior of either electrode independently of the other. Auxiliary electrodes in these applications are a valuable tool to the researcher since he can carry out his studies without having to dismantle the cell construction.
Provision must of course be made in the sealed assembly of the current-producing electrochemical cell for making external electrical connection with the auxiliary electrode. Such means usually comprises a separate or third terminal connection in the sealed assembly in addition to both the positive and negative terminal connections. Basically the same type of construction is used for the third terminal connection regardless of whether the auxiliary electrode is employed as an oxygen or hydrogen sensing electrode or voltage reference electrode.
The principal object of this invention is the provision of a flat alkaline cell construction of the character described in our above-referred to copending application wherein there is provided both positive and negative terminal connections in the walls of the plastic film envelope and a third terminal connection for an auxiliary electrode.